The object of this research is to relate growth hormone and prolactin metabolism to mouse mammary tumorigenesis. The investigation particularly utilizes specific homologous radioimmunoassays of these hormones for their concentrations in pituitary glands, serum and other fluids. The natural history of secretory patterns is being studied in strains having differing incidences of these tumors, in foster-nursing studies, and in pharmacological studies and in relation to the nutritional state. Possible effects of the mammary tumor virus and of chemical carcinogens are of special interest.